Monolithic circuit manufacture and photoresist exposure technique utilized therein



Mr! v =1 q! vn' 5197A KR 3e524939 [72] Inventor Brian Sumter-s IPnughkeepsie New York [Ill AppllNni 594,752 {22] Filed Nov. 16.1966Patented Aug. 53. U970 {73] Assignee international Business Machinesjorporatian Armontt New York 5050-! a Corp? New York [54] MUHOMTHICQKRCUH MANUFA-CT-UEMZ AND PHOTORESKST EXPOSURE TECHNEQLE UTZMZED THEEEENl2. Ciaims, 52 Drawing Figs.

[52] U.S.Cl A. Ha/l2. 346/107 350/385, 355/ l5ll EnLCI EGlc57/08 Fieidol'Search 350/285; /1. 12:346/l07. l08:355/40 gm 84 i plate heft-neatPrimary- 1;'.\'a.'nimr- John M4 Hnrnn Assismu/ Examiuw-- Richard A.\Vintcrcnrn .-illunie ri-ianifin and Jancin ABSTRACE: An apparatus' forexposing the phutnrcsist coating on a semicnnductur wafer to formthercnn the metallic interconnection pattern which wires up a pluralityof network cnmpnnerits to lnrn a single monolithic integrated circuit Alight he' 'trge angalgir plate pr ly sniall disw he niTTheangtiliiriliriplafehicnts ol the rrfii'fim plateare contrullcd by a computerwhich determines the configuration nfthc wiring interconnection patternfrom test data designating which network components are acceptable foruse in the pattern.

DIGITAL TAPi w DlElTM M T0 HEADER CO EFUTER AElALO CiEiVERTEH DIGITALMMLGG CWWERTER PRiOR ART This invention relates to a technique forexposing a photoresist coating, and to the manufacture of monolithiccircuits utilizing said technique.

ln the manufacture of large-scale integrated circuits wherein a numberof network components are formed and interconnectcd on a single unitarysemiconductor wafer. the major problem arises from the yield. That is. asubstantial portion of the network components ofany wafer specimen are.in the present state of the art, inevitably defective and incapable ofuse. lt is therefore the present practice to form the wafer withredundant network components which may be either suhcircuits orindividual devices. The various network components are first tested todetermine which are acceptable and which are defective. An individualwiring interconnection pattern is then determined for each particularwafer so as to util ize and interconnect only those network componentswhich are acceptable. while those components which are defective are notused in the final overall interconnected circuit.

This so-called "discretionary wiring" technique of providing anindividualized wiring interconnection pattern for each wafer is. in thepresent state of the art. difficult. time-consuming and expensive. andit is this problem with which the present invention is primarilyconcerned. Priorto the advent of the present invention there were onlytwo techniques in general use. The first technique involves the layoutand fabrication of an individual photoresist exposure mask for eachparticular wafer. This requires the initial creation of artwork fromwhich the mask is formed by a photoreproducti e process. if performedmanually this technique is laborious. time-consuming and subject tohuman error. lf performed with design automation apparatus there isinvolved a heavy initial investment in the design and construction ofthe equip ment Furthermore. once the mask is used to expose thephotoresist coating on the particular wafer for hich the mask \\';iscreated. the mask is then useless and the large expense iiivtil\cd inits fabrication is allocated to a single wafer so as to greatly increasethe cost ofmanufacture ofthe latter.

The other technique of the prior art is to expose the pho toresistcoating on the wafer with a light beam so as to form a wiringinterconnection pattern by relative displacement ofthe light beam andthe wafer. Dueto the literally microscopic dimensions involved in theinterconnection pattern it is extremely difficult to provide relativedisplacement of the light beam and the wafer with sufficient accuracyand precision. This is because the mechanical devices for displacing thewafer or the light beam inherently involve inaccuracies due to backlash.wear. and manufacturing tolerances in the screw threads. gears and otherengaging parts.

These deviations in the mechanism provide relatively large inaccuraciesin the locus traced by the light beam because of the lack ofdeamplification in the conversion of mechanical movement to opticaldisplacement. That is. the displacement of a mechanical part a givendistance results in an approximately equal displacement of the lightbeam. Hence. the errors and inaccuracies of the light beam locus are ofthe same order of magnitude as those of the mechanical parts and'it isdifficult to maintain the high degree of precision required for themicroscopic dimensions of the wiring pattern of a monolithic integratedcircuit.

These disadvantages of the prior art are obviated in the presentinvention by a novel apparatus for exposing the photoresist coating onthe semiconductor wafer so as to form thereon the metallicinterconnection pattern which wires up the various network components toform a single circuit. In the preferred embodiment of the inventiondisclosed herein for purposes of illustration. the light beam istransmitted through a retracting glass plate before it strikes thephotoresist. A relatively large angular displacement of the glass plateprovides. in accordance with Snells law of refraction. a relativelysmall displacement of the light beam. This provides a largedeamplil'ication in the conversion of the movement of the mechanicalparts to displacement of the light beam, and

hence any errors and inaccuracies in the mechanism result in muchsmaller deviations in the locus or path traced out bythe light beam.

The angular displacements of the refracting plate may be controlledautomatically in response to test data designating which networkcomponents are acceptable so as to determine the configuration of thewiring interconnection pattern to be traced out by the light beam. Inthe preferred embodiment disclosed herein the test apparatus appliesprobes in succession to the various network components on the wafers sothat elec- -'tric.al test signals may be applied thereto. The responsesofthe network components are evaluated by a digital computer and thenthe resulting data is printed out in the form of punched paper tape. Thelatter is then fed to a tape reader constituting an input for a secondcomputer having stored in its memory programmed information as to thecircuit to be formed on the particular wafer. The second computer thenprocesses the test data along with the circuit information anddetermines an individual wiring interconnection pattern which is to heformed on that particular wafer. To realize this pattern the computergenerates digital instructions which are converted to analog drivesignals by a digital-to-analog converter. These analog drive signals arethen transmitted to servomotors which angularly' displace the refractingglass plate with the required sequential movements so as to cause thelight beam to trace out the required locus on the photoresist coating ofthe wafer. The exposed photoresist is then developed and a metallicwiring interconnection pattern is formed in conformity therewith byetching. evaporation or other conventional technique. Alternatively. thewiring pattern may be formed by diffused conductive regions in thewafer.

lt is therefore a primary object of the present invention to. provide anovel technique for exposing a photoresist coating on a workpiece bymoving a light beam thereover in a precisely dimensioned locus andhaving the advantages set forth above.

An equally important object is the utilization of said expos ingtechnique to provide an improved method for the manti facture ofmonolithic integrated circuits with greater economy'. yield. speed andaccuracy than heretofore.

Instead of utilizing the subject exposing apparatus to expose thewaferdirectly as in the preferred embodiment disclosed herein. it may be usedto expose a photographic film to provide artwork from which a mask maybe made. The mask may then he applied to the wafer to expose the latter.This mask making technique may be utilized with either theabovedescribed discretionary wiring approach or with a fixed wiringinterconnection pattern suitable for small-scale integration or.high-yield wafers.

Other objects and advantages of the present invention are eitherinherent in the specific embodiments of apparatus and methods disclosedherein or will become apparent to those skilled in the art as thedetailed description proceeds in connection with the accompanyingdrawings wherein:

HG. l is a schematic view ofa conventional system for testing thecomponents of the masters-lice pattern formed in a monolithic integratedcircuit wafer;

FTC. Z is a schematic view of the system in accordance with the presentinvention for exposing the photorcsist coating on the wafer so as toform a wiring interconnection pattern;

FIG. 3 is a top plan view of the support for moveably. mounting theglass refracting'plate-which deflects the light beam;

HG. 4 is a vertical section view taken substantially on line 4-4 of FIG.3'.

HS. 5 is a schematic view of the geometrical optics involved in theapplication of Snell's law ofrefraction to the sub-. ject invention;

FIG. 6 in an enlarged schematic view of a portion of a monolithiccircuit wafer showing the wiring interconnection pattern connected tothe acceptable circuit components;

HO. 7 is .i stliciiiatic tioss sectional \ls'\\ ol a portion of .i ualiioi otht-i uoiLpii-tc having .i ruclallit l.i\ci and .i plio toicsisl shlllll)! tltt'ici ii,

l-Hi. X shims tlic lalr'iit iiiiagc lt llllttl Ill thipliiitoicsist.illci tlii walii iil PHI. 7 isrstiost-il.

Hti. sliovss lllL' \salci .iltci the cspost-tl pliotoicsist li.is lieeiideveloped and the iiiicsposcd poitioiis tlici'cof |L'lllU L'Lli HG. l lslio\\s tltc .itei alter the etching step has reiiio\ ed those portionsof the metallic la ver not protected h the devcloped phoioresist;

HG. ll shows the water after the photoresist has heen washed awa)leaving the metallic interconnection viiring pattern;and

H6. 12 is a schematic vie shutting the use of a modified light sourceemitting a pluralit of collimated light hcams for tracing simultaneous!a pluralit of identical latent images on a single wafer Referring nosvto the drawings in more detail. and first to HG. l. there is shovsn aComentional .s vstem for testing the various circuit elements or othercomponents of the master- .slice pattern formed in a monolithicintegrated circuit nafcr so as to determine vshich of the components areacceptahlc and which are defecti e. The wafer is indicated h thereference letter W and is mounted h an arrangement including structuralelements 36' to -33 inclusive and dcscrihed in the hi-lovi descriptionof HG. 1 herein corresponding elements haw applictl thereto the samereference numerals \vithout the prime s) mhol. l he test nodes on ilie\sal'ei surface are coritactcd hv the respective tecler elements it of apluralit of test probes l2 connected h leads l3 to a te ter apparatus 14The latter generates input test signals liicli .irc applied to the inputnodes of the wafer components and receive the response signals from theoutput and other nodes thereof. 'l he response signals are evaluated h adigital computer l-la hich determines which of the afer components areacccpta hle. This data ma then he recorded h a tape punch l5 .so as tocreate a paper tape constituting a record of those \vafer componentswhich mav he utiliI cd in the final inter-connected netuork to he formedon the wafer W Referring no to MG. 2 there is shoiin the iio\elapparatus for exposing the photoresist coating on the wafer W so as toform thereon a \sirtng interconnection patternv The apparatus comprisesa tape reader It) for reading the test data from the paper tape formed htape punch l5v The data is then fed to a digital computer 17 lilClt isprouded vvitli a design dUlUItttl' tion program including information asto the circuits and net- \vorks to he formed on the wafer W. Computer l7anal \7e the test data in conpinction \Hlll the circuit or networkinforiiiation and then determines a wiring pattern to interconnect theacceptahle components.

Computer l7 accordingl} generates digital instructions which are fed toa digital-to-analog cornertcr Iii iliich con verts these digitalinstructions into analog drnc signals transniitted through leadsl9a.2ila to scriomotors 19.29 TcspcL tivel The latter angularlv displacea glass plate 22 \iliich refracts a light beam 22 radiated from a lightsource 23 so as to move the light heani 22 over the photoresist coatingon the upper surface ofisafer W and therehs expose the requiredtllterconnection pattern on the latter.

The details of tape reader to. digital computer l7 and the designautomation program therefor. and drgital-to-analog converter l8 are notdisclosed herein hecause the are well known in the art and form no partof the present invention. Hovs ever. the apparatus for rum mg lightheani 22 \sith respect to wafer W is hcliewd to he novel and the detailsthereof will no he describedv The wafer W is aligned h pins 36 attachedto the upper .siir face of a first slide member .17 slitlcahl mountedwithin a dovetail slot 38 formed in a second slidc ntciiil ci It)\iliich is in turn slideahlv mounted itliin opposite! facing -sllitpc'tlgrooves 40.4] formed in a pair oi support niciiihcrs 42.43 secured tothe upper surface of a hase plate s id. l irst slide nicmher 37 isntrncahlc in a direction orthogonal to the lll Ill

direction Hf IlltHc'llli Hl of second slide nicnil cr .W. ll\ cposiiioii ol the lattvi is .idtiistcd h .i maiuiallv ad iistal lc sci'cu5 tliicadctl \sitltiii .i ll'dL'lstl -46 sccuicil to l asc platc -34.and tltg; position ol l'iist slide iiicniliei 37 is \llllllttll)ailuistctl l .l screw 47 threaded vs itliin a bracket 1H scciu Ltl tosecond slidc rncinhei' A). lhc position of water W llttl} thus heprecisely determined.

As sho n in HUS. I to 4. sci'voniotor 20 has an output shaft 50 to whichis secured a \s'orni gear 5! engaging an aicuale gear segment 52. Thelatter is secured to the outer end of an arm 53 ha\ ing its inner endformed integral with a rectangular frame indicated generally hy thereference numeral 54. The latter comprises a pair ol'parallel frameelements 55.56 interconnected at their ends h v transversely extendingframe elements 57.58. Frame 5-2 is mounted for angular displacementahout the axis of a pair of coaviallv aligned pivot pins 59.5% securedto the upper ends of a pair of respective hrackets 60.6fm attached tothe upper surface of a support plate 61.

The reference numeral 62 indicates generally a yoke coniprising anintermediate portion 63 having integrally formed at its opposite ends ofpair of transversely extending arms 64.65 each having at the outer endthereof a hearing 66.67 within which is secured a pivot pin (v8.69rotatahl mounted within suitable hearing openings formed in therespective frame elements 57.58. The inner ends of pivot pins 63.69 aresecured h cotter pins 70.7] to a rectangular frame support Zia cttendingpenpheralls around and secured to the marginal edges of the glass plateBl. A pair ofspacer washers 72.73 surround pins 68.69 hetuecn respectncframe elements 57.58 and the glass support 21a The latter together \sithglass plate 2i and \okc 62 are thus mounted for angular displacementabout the common axis of hearing pins 622.139.

The angular position of this assemhh is controlled by servomotor 19having an output shaft 75 formed ilh a worm gear 76 engaging an arcuategear segment 7-l attached to the intermediate portion (:3 of yoke 62.Servomotor E9 is secured h v a strap 77 to a support 78 attached to theupper end of bracket 60a. Glass plate 2] ma thus he angularlv displacedahout the orthogonal uses of the pivot pin pairs 5959a and 68.69.

Light source 23 is shovvn schematically in FM]. 2 and com prises a cvlindrical housing tit) having a top cover til mounting a light hulh 82.Light from the latter is collimated to form a narrow light beam 22 h acondenser 84 and a pair ofaperttire plates 35,86 ha\ ing a pair ofcoaxially aligned apertures 87,83 through which the light heam 22 isemitted. A shutter 89 is rnoveahle either to a closed position hlockinglight heain and preventing it from striking \vat'er W or to the openposition shown in MG. 1 \vherehy light beam 22 is permitted to esposethe photoresist coating on water W. The movements of shutter 89 areeffected hy a servomotor 9t) controlled by computer l7 and actuated byanalog signals from converter 18 connected to servomotor 9t) h v a line9!.

HG. 5 shovvs the geometrical optics involved in the operation ofdisplacing light heaiii 22 hy angular movements of refracting glassplate 21. For a constant plate thickness and refractive index. tiltingof plate 21 in a plane normal to light heam 22 causes the refracted heamto he displaced from the position it would have if plate Zll were normalto the ray. For esarnple. in a l" thick plate of transparent materialwith a refractive index of T5. as in the case of silica and mercurylight at about 3500 A should he A 20.3. the displacement of light beam22 is approximately 0.606" per dcgrcc oftilting of plate Zl. Thissensitivity is directly proportional to the thickness of plate 2!. andis little changed by a variation in refractive index.

A displacement of up to 0.150" can he ohtained for tilt angles of 25 orless. angles at which loss of light intensity hy reflection is small,and which in any case could he compensated for h the additionofanti-rcllcction coatings to plate 23. It is further estimated thatlight hcains as small as 0.0001" in diameter can he colliniated withoutintroduction of interference effects. This diameter is 3 times thewavelength of the lightv liotli this diainctcr and,.thc scan dimensionsare those presently envisaged as being used in fully integrated ciruitry. it is further estimated that the rotation or tilting of plate 21in both the X and Y directions can be controlled to between 0.1 to 0.01.possibly permitting a line width of 0000]" located with a locationaccuracy of between 0.0006" to 0.00006".

Simple mechanical translation of the whole system can be used to move itfrom one set of interconnections to the nest. when it has reached itsmaximum throw by tilting. This maximum throw. is. of course. relatedinversely to thethickness of plate 21. and thus inversely to thesensitivity. and directly to the loss of intensity due to reflectiontolerable at high angles of incidence. I

- The sensitivity may be calculated as follows. where the dimensionsshown in MG. 5 are defined to be;

OA=thicknr ss of plate BC tr-nnslation of beam angle AOC angle ofincidcnc :1 angle AOB -anglo of rcfraction=r u I'PhZLCliVU index andsince Plotting BC. the movement of the light spot. against values of i.the angle of incidence for values of 5. l. l. and keeping ()A=i" and;1=i.5 there is obtained .iitiii" .000"

Therefore the sensitivity under these conditions is approximately0.006"/degrcc of rotation.

H6. 6 shows schematically an enlarged iew of a portion of atypicalmonolithic circuit wafer W. The curcuits are located within the squareareas designated by the dashed lines. those circuits having been foundgood or acceptable being designated by the reference letter G and thosehaving been found defective being designated bythe reference letter D.The acceptable circuits G are shown interconnected by a metallic wiringpattern comprising vertical metallic conductive strips V and horizontalstrips H. The defective circuits D are not used and the wiringinterconnection strips V. H are not interconnected therewith so that thedefective circuits l) are omitted from the final network formed on waferW.

A preferred method of forming the metallic wiring intcrcon ncctionpattern will now he described with reference to FIGS. 7 to H inclusive.lhc water \V. usually in the form of a monolithic silicon block.is'pi'ovidcd with a metallic layci I over which is superimposed aphotorcsist coating l. After the exposure step by the apparatusdescribed above. there is formed iii photoresisi coating l a latentimage comprising cs posed areas F. as sho\\ n in Flt X. Photorcsistcoating l is then developed and the unexposed portions of coating P areremoved to leave the exposed portions l-l as shown in MG. J. An etchant.such as hydrochloric acid. is then applied so as to etch away and removethose portions of metallic layer M which are not protected by theexposed remaining portions E of photoresist coating Pubereby leavingmetallic strips forming theuinterconnection pattern I as shown in MG.10. The exposed developed photoresist portions E are then removed toleave the interconnection pattern I upon the upper surface of wafer Wasshown in MG. ll.

In FIG. l2 there is shown schematically an arrangement for exposingsimultaneously a plurality of intraconnection patterns to form aplurality of identical circuit elements which may then be interconnectedin the manner describedabove and shown in HG. o. A light source 123.condenser and aperture plates 126. 127 form a plurality of parallellight beams 222 which are refracted and displaced by a glass plate 2! soas to expose simultaneously a plurality of identical patterns on thewafer W. Plate l2l may be angularly displaced about orthogonal axes bythe apparatus described above with respect to NOS. 2 to 4 inclushe.

As shown by the calculation above. the movement ofa light beam isrelated only to the angle of the glass retracting plate relative to theincident beam and not to the position of the latter. Thus the pluralityof collimated beams I22 are retracted and displaced by identical amountsin response to the angular displacement ofplate l2l.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the invention.

l claim:

1. An apparatus for exposing a workpiece having a photoresist coatingthereon and comprising:

a workpiece holder for aligning and maintaining the workpiece in apredetermined position.

a light source for radiating a light beam to the photoresist coating ofthe workpiece.

a light deflecting member to deflect the light beam in response tomovement ol'the member said light deflecting member comprising aretracting element through which the light beam is transmitted in itspath front the light source to the workpiece.

a support for movably mounting the light deflecting member.

an actuating device operable to move the light deflecting member. and

a control system for directing the operation of the actuating device tomove the light deflecting member with a predetermined displacement andthereby deflect the light beam to expose a desired pattern on thephotorestst coating 2. An exposing apparatus as recited in Claim l,wherein said retracting element comprises a transparent glass platehaving substantially parallel planar opposite surfaces and angularlyinclined with respect to the light beam.

3. An apparatus as recited in Claim 1. wherein said-light sourcecomprises means for collimating said light beam.

4. An exposing apparatus as recited in Claim 1. wherein said lightsource comprises means for radiating a plurality ofparallel light beamsfor the simultaneous exposure of a plurality of identical patterns onthe photorcsist coating oftlte workpiece.

5v An apparatus as recited in Claim 1. wherein said support for niovablymounting the light deflecting member comprises means for mounting themember for angular displacement about a lirst ans of rotation and mcansfor mounting the member for angular displacement abotit a second axis otrota tion substantially orthogonal to said first axis.

6. An apparatus for exposing a workpiece having a photoresist ciiittingthereon aitd comprising:

a workpiece holder l'or aligning aitd maintaining the workpiece in apredetermined position. .i light source for iadiatmg a light beam to thepliotoresist coating ol the workpiece. a light deflecting member todeflect the light beam iii response toniovenient ol'the member. asupport for movably mounting the light deflecting member. an actuatingdevice operable to move the light deflecting member. and a controlsystem for directing the operation oi the actuating device to move tltelight deflecting member with a predetermined displacement and therebydeflect the light beam to expose a desired pattern on the photores'istcoating, said control system comprising an input de ice for receivingdata as to the characteristics ofthe workpiece. a memory device forstoring information as to the circuit pattern configuration to beexposed on the workpiece l photoresist coating. and

I a control means lor directing the operation of said actuating devicein response to both said data and said information.

7. An exposing apparatus as recited in Claim 6. wherein said controlmeans comprises a digital computer for analyzing said data and saidinformation and for formulating digital instructions in responsethereto;

I a digital to analog converter for converting said digital instructionsinto analog drive signals. and

1 said actuating device comprising motor-driien means energizeable bysaid analog drive signals.

8v An apparatus for exposing a workpiece having a photoresist coatingthereon and comprising a workpiece holder for aligning and maintainingthe workpiece in a predetermined position.

a light source for radiating a light beam to the photoresist coatingot'thc workpiece.

a light deflecting member to deflect the light beam in response tomoi-emcntotthe member.

a support for movably mounting the light deflecting member.

(ill

an acfiiuating device operable to mow the light deflecting member acontrol system for directing the operation of the actuating device tomove the light deflecting member with a predetermined displacement andthereby deflect the light beam to expose a desired pattern on thephotoresist coating. and

a testing apparatus for preliiiiiriatily inspecting the work piece to beexposed and lor generating data as to the characteristics olthe specificworkpiece.

said eontrol system including means for receiving said data and fordirecting the operation of the actuating device in response to saiddata.

9. An apparatus for exposing a monolithic circuit wafer having aphotoresist coating thereon and comprising:

a wafer holder for aligning and maintaining the wafer in a predeterminedposition.

a light source for radiating a light beam to the photoresist coating ofthe water.

a light deflecting member to deflectcthe light beam in response tomovement of the member said light deflecting member comprising aretracting element through which the light beam is transmitted in itspath from the light source to the wafer.

a support for movably mounting the light deflecting member. and

an actuating device operable to move the light deflecting member 10. Anexposing apparatus as recited in Claim 9 wherein said retracting elementcomprises a transparent glass plate having substantially parallel planaropposite surfaces and angularl inclined ith respect to the light beam.

ll. An exposing apparatus as recited in Claim 9 wherein said lightsource comprises means for radiating a plurality of parallel light beamsfor the simultaneous exposure ofa plurality of identical circuitinterconnection patterns on the photoresist coating olthe wafer.

12. An apparatus as recited in Claim 9 wherein said support for inoiablymounting the light deflecting member comprises means for mounting themember for angular displacement about a first axis of rotation and meansfor mounting the member for angular displacement about a second axis ofrotation substantially orthogonal to said first axis.

